Fix #100: Allow unionTagName to be part of the record type definition when deserializing union of records

Author: Tarmil

src/FSharp.SystemTextJson/Union.fs

@@ -359,7 +359,8 @@ type JsonUnionConverter<'T>
     let readInternalTag (reader: byref<Utf8JsonReader>) (options: JsonSerializerOptions) =
         if namedFields then
             expectAlreadyRead JsonTokenType.StartObject "object" &reader ty
-            let struct (case, usedDocument) = getCase &reader
+            let mutable snapshot = reader
+            let struct (case, _usedDocument) = getCase &snapshot
             readFieldsAsRestOfObject &reader case false options
         else
             expectAlreadyRead JsonTokenType.StartArray "array" &reader ty

tests/FSharp.SystemTextJson.Tests/Test.Regression.fs

@@ -25,3 +25,14 @@ let ``regression #87`` () =
     Assert.Equal("A.B was expected to be of type String, but was null.", ex1.Message)
     let ex2 = Assert.Throws<JsonException>(fun () -> JsonSerializer.Deserialize<A>("""{ "A": null, "B": "a" }""", options) |> ignore)
     Assert.Equal("A.A was expected to be of type Int32, but was null.", ex2.Message)
+
+type R100Rec1 = { Case: string }
+type R100Rec2 = { Case: string; other: int }
+type R100U = Rec1 of R100Rec1 | Rec2 of R100Rec2
+
+[<Fact>]
+let ``regression #100`` () =
+    let options = JsonSerializerOptions()
+    options.Converters.Add(JsonFSharpConverter(JsonUnionEncoding.InternalTag ||| JsonUnionEncoding.UnwrapRecordCases))
+    Assert.Equal(Rec1 { Case = "Rec1" }, JsonSerializer.Deserialize("""{"Case":"Rec1"}""", options))
+    Assert.Equal(Rec2 { Case = "Rec2"; other = 42 }, JsonSerializer.Deserialize("""{"Case":"Rec2","other":42}""", options))